nerve biopsy

I have had symptoms of non length dependent small fiber neuropathy (muscle aches and burning, tingling, stabbing sensations in my legs; thighs worse than calves and only feet at times with later onset of pressure sensations in my hands and tingling, pressure in midzone of face) for about a year. Have had EMG, Autonomic testing and skin biopsy, but nothing came back as abnormal. My neurologist still thinks that I have SFN and suspects that it may be due to autoimmune disease. I do have + ANA and low complement levels with borderline low white blood cell count and anemia. I don't have enough abnormal findings to make a specific diagnosis although my rheumatologist thinks I could have vasculitis or maybe lupus. The neurologist is recommending nerve and muscle biopsies. I am very afraid that the biopsies will be negative like all of the other tests. Is it possible to have SFN with no concrete proof? What is the probability that the nerve and muscle biopsies will provide a diagnosis?